Cutting tools experience wear while machining as a result of a variety of thermo-mechanical mechanisms. Heat generated due to plastic deformation of the material being cut that is inherent with chip formation, coupled with the rubbing/friction between the chip and the machined surface against the tool, cause the tool to become hot during machining. These contacts—chip-tool and tool-workpiece—are the primary heat sources acting on the tool and, furthermore, they are of a stationary nature with respect to the tool and thus cause the tool to get quite hot. If the tool gets too hot, it can soften and suffer plastic deformation. Under more normal operating conditions, though, the tool does not soften but the natural wear rate does increase as the tool temperature increases. The primary process variables that affect tool temperature include cutting speed, feed rate and depth of cut, in decreasing order of the strength of their typical effect. Increasing any one of these variables leads to a greater material removal rate, which is desired, but also an increased wear rate which reduces tool life as measured in both time and volume of material removed.
In order to moderate tool temperatures to enable either a higher material removal rate while maintaining an acceptably low wear rate (and thus an acceptably high tool life), or to reduce wear rate (and thus increase tool life) for a given material removal rate, metal-working fluid is usually applied to the process. That fluid is often referred to as a “coolant” or “cutting fluid”. It provides cooling as well as some lubrication, the latter in particular as it relates to flushing and evacuation of chips from the cutting zone. The cooling effectiveness of the cutting fluid is increased when there is an improvement in the rate of heat transfer from the process heat sources to the cutting fluid. Direct access to the heat sources is precluded since the heat sources are the highly stressed mechanical contact patches between the chip and the tool and the tool and the machined workpiece surface. Thus, this heat transfer mechanism involves initially the conduction from the heat sources through the tool followed by convection from the tool to the cutting fluid. Thus, the overall heat transfer into the cutting fluid is improved by one or both of the following: (1) higher velocity of flow of the cutting fluid over the tool, hence increasing the convective heat transfer coefficient of that fluid-solid interface and (2) minimizing the distance from the cutting fluid contact with the tool relative to the heat sources acting on the tool—the zone of chip contact with the rake face and the zone of flank face contact with the machined workpiece surface; the flank face contact includes the flank wear land that forms over time as the tool wears.
Flood cooling is a simple and common way of applying cutting fluid to the process. Growing in popularity is the use of streams or jets of coolant targeted at the cutting zone. These jets are often high in pressure (typically 500-1,000 psi is currently considered high pressure in these applications). One example is shown in FIG. 1 which shows a cutting tool, specifically an inserted end mill, comprising a Shank 1 and a Tool Body 3. Each cutting Insert 4 on this inserted end mill Tool Body 3 has a Nozzle 103 that produces a jet (not shown). Each Nozzle 103 is fed with cutting fluid from a central hole running down the axis of the cutting tool resulting in a jet of coolant spraying onto the rake face of the respective Insert 4. Rotating cutting tools, including but not limited to these end mills and other types of milling cutters, require a machine to have “through-spindle” coolant delivery, which has become common.
As another example in a cutting tool for lathe (turning and facing) processes, U.S. Pat. No. 4,621,547 and a commercial tool seen in FIG. 2 show how Jets 104 (typically one to three) can be exhausted toward the chip contact with the Rake Face 105 by forming a Nozzle 103 via a hole in the Insert Clamp 106 or a channel on the underside of the Insert Clamp 106 that then sits on the Rake Face 105 to create a fully encircled exhaust port (equivalently a nozzle) for the cutting fluid. In both cases (those illustrated in FIGS. 1 and 2) these Nozzles 103 are generally 1 mm in diameter (0.79 mm2 cross-section) and larger. Photographs of actual fluid exhausting from the pair of exhaust ports on the cutting tool depicted in FIG. 2 show that the jets immediately diverge into wide sprays of cross-section much larger than that of the exhaust ports as opposed to non-diverging streams of cross-section similar to that of the exhaust ports.
The intent with the approach shown in FIGS. 1 and 2 is to employ a focused high-pressure jet to penetrate the narrowly exposed and often moving (e.g., with cutting tool rotation) chip-tool contact zone. By increasing the jet cross-sectional size and/or pressure, said penetration of the cutting fluid is facilitated by slightly bending the chip upward due to the force the coolant jet exerts on the backside of the chip, even to the extent of assisting in breaking the chip, which is also desirable and typically a significant goal of such systems (though chip breaking is not as much an issue in milling processes due to the natural chip breaking that occurs due to the inherently intermittent nature of milling). A larger jet can provide a greater force on the chip to improve cutting fluid penetration (by prying the chip slightly away from the rake face) and increase the likelihood of breaking the chip. The magnitude of the jet's force increases with the volumetric flow-rate and pressure (which translates into fluid velocity). In this sense, it is desirable and attempted to make the jets as large as possible. However, the maximum sizes of the nozzles and thus the resulting jets are limited in part by the space available to make the nozzle and also by the coolant pump capabilities. For instance, as a nozzle gets larger in cross-section the volumetric flow-rate gets higher for a given pressure. Once the volumetric flow-rate exceeds the pump's capacity, the pressure drops and thus a limit is reached on the force applied by the jet. When a nozzle is smaller and a pump's volumetric flow-rate capacity is underutilized, pressure is maintained but again with the dependence of the jet's applied force on volumetric flow-rate in addition to pressure, the force applied by the jet is limited. In other words, it is desirous under this approach to have nozzles/jets that are large and pumps that have high pressure and high volumetric flow-rate capacity; pumps of that sort are costly and a limitation.